In the medical field, reduction in the amount of processing solution discharged is keenly demanded in recent years from the viewpoint of environmental conservation and space savings. To cope with the demand, technology relating to the light-sensitive heat-developable photographic material (photothermographic material) for use in medical diagnosis or photographic use, which can be effectively exposed by a laser image setter or a laser imager and can form a sharp black image having high resolution and sharpness, is required. Such a light-sensitive heat-developable photographic material can dispense with solution-based processing chemicals and afford users a more simple heat-developing system freed from impairment of the environment.
As the heat-developable light-sensitive material which seems to satisfy the above-described requirement, those where light-insensitive organic silver salt (particularly a silver salt of a long chain alkane carboxylic acid, such as silver behenate) and a reducing agent for silver ion are made present together with light-sensitive silver halide in a relatively hydrophobic binder, are known.
By adsorbing a spectral sensitizing dye to the silver halide used here, the light-sensitive material can be spectrally sensitized to light of various wavelengths.
In the silver halide photographic emulsion used in normal silver halide photographic light-sensitive materials, silver halide fine crystals are dispersed in a hydrophilic colloid represented by an aqueous gelatin solution, however, the light-sensitive silver halide in the above-described heat-developable light-sensitive material using a hydrophobic organic silver salt is present in a hydrophobic organic solvent almost free of binder and water. The spectral sensitizing dye used in spectral sensitization of normal silver halide emulsion is generally adsorbed with difficultly to silver halide fine crystal in an organic solvent, accordingly, when it is used for the spectral sensitization of silver halide of the light-sensitive material using an organic silver salt, there arise problems such that the sensitivity is not sufficiently high and the capability changes during storage, such as extreme reduction in the sensitivity or increase of fogging.
The reduction in sensitivity is a serious obstacle particularly in achieving high sensitivity necessary for increasing the scanning speed upon scan exposure with a semiconducting laser light which has been rapidly advanced in recent years. Further, desorption and decomposition from the silver halide fine crystal is outstanding in the case of a spectral sensitizing dye for the infrared region, and improvement in this respect is demanded.
On the other hand, the semiconducting laser technology marking a rapid progress in recent years has succeeded in miniaturizing the output device of an image for medical treatment. Naturally, technology of infrared ray-sensitive light-heat silver halide photographic materials has been developed. The technology of spectral sensitization is disclosed in JP-B-3-10391 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-6-52387, JP-B-5-341432, JP-A-6-194781 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-6-301141, and the antihalation technology is disclosed in JP-A-7-13295 and U.S. Pat. No. 5,380,635. According to these techniques, the sensitizing dye or the antihalation dye in the light-sensitive materials postulating exposure to infrared ray can be greatly reduced in the absorption of visible light, and a substantially colorless light-sensitive material can be easily produced.
The dye which absorbs infrared ray and effects spectral sensitization is generally high in the HOMO and has strong reducing ability, accordingly, it reduces silver ion in the light-sensitive material and the light-sensitive material is liable to be badly fogged. In particular, when the light-sensitive material is stored under high-temperature and high-humidity conditions or over a long period of time, problems accompanying the outstanding change in capability are caused. If a dye having low HOMO is used for preventing the deterioration of storability, the LUMO becomes relatively low, as a result, the spectral sensitization efficiency is reduced and the sensitivity is lowered. These problems of sensitivity and storability are not limited to wet photographic materials but more notable in the heat-developable photographic material to which the present invention relates.
The reducing ability of dye is naturally greater as the amount of dye is larger. However, if the amount of dye is reduced, light entering the light-sensitive material cannot be satisfactorily absorbed and lack of sensitivity results. Particularly, in the heat-developable light-sensitive material using a binder having high lipophilic property, as described above, adsorption of a dye to silver halide as a light-sensitive element is weak and unless a sufficiently large amount of dye is added, the sensitivity becomes deficient.